Over recent years, image forming apparatuses based on electrophotographic methods have been used as common copiers and printers in offices to print documents as well as for simple copying, and moreover have expanded their application to the area of preparing printed materials used outside offices, specifically to the print-on-demand (POD) market, which is included in the quick-printing market, since variable information from electronic data is readily printed. Accordingly, various copiers and printers have been installed in offices, resulting in increased power consumption as a whole.
In the POD market, a practical value is sought not for copying but for printed materials themselves. Therefore, these printed materials have needed to be produced with high image quality.
To obtain printed materials exhibiting high image quality, it is known that it is effective to decrease the particle diameter of a toner. To realize the above, various types of so-called chemical toners have been proposed. Such chemical toners are produced via a method to carry out granulation in an aqueous medium, and therefore have the advantage that toner particles of relatively small particle diameter are obtained with enhanced uniformity, compared to those produced via a pulverization method.
In contrast, to obtain printed materials exhibiting high image quality by providing enhanced glossiness with no occurrence of offset phenomena during fixing, it is known that it is effective to use polyester resins as binder resins constituting toner particles.
Then, as a method to prepare a toner of decreased particle diameter using a polyester resin, there is proposed a method to obtain toner particles wherein a polyester resin is dissolved or dispersed in a solvent and dispersed in an aqueous medium to form oil droplets, followed by desolvation.
As catalysts used to synthesize a polyester resin via polycondensation, catalysts composed of tin compounds such as dibutyltin are commonly used (for example, refer to Patent Document 1).
However, in this method, a colorant is dissolved or dispersed in a solvent together with a polyester resin, followed by granulation. Therefore, there is noted such a problem that dispersibility of the colorant in toner particles obtained is decreased.
Further, tin compounds used as catalysts as described above are organotin compounds featuring a structure wherein an aliphatic substituent is bonded to a metal (tin). It has recently been pointed out that such organotin compounds are problematic in environmental soundness and safety. Therefore, use of these catalysts is being reviewed.
From the viewpoint of such environmental consciousness, there have been recently proposed metal catalysts including titanium catalysts such as titanium halides, titanium diketone enolates, titanium carboxylates, titanyl carboxylates, and titanyl carboxylate salts; germanium catalysts; and aluminum catalysts (refer to Patent Documents 2-4).
However, when a toner based on a polyester resin obtained employing such a specific metal catalyst is used over a long term period, there is produced such a problem that image density of visible images formed is gradually decreased.    [Patent Document 1] Unexamined Japanese Patent Application Publication (hereinafter referred to as JP-A) No. 2005-173570    [Patent Document 2] JP-A No. 2004-126544    [Patent Document 3] JP-A No. 2005-91696    [Patent Document 4] JHP-A No. 2005-91525